1. Technical Field of the Invention
The present invention relates to a method of producing silicon carbide articles.
2. Description of the Related Art
Silicon carbide (SiC) or moissanite is a ceramic compound consisting of carbon and silicon. SiC was accidentally discovered by Edward G. Acheson, an assistant to Thomas Edison, about 1890, when he was running an experiment on the synthesis of diamonds. Acheson thought the new material was a compound of carbon and alumina, present in the clay, leading him to name it carborundum, a name that is still being used on some occasions. SiC occurs naturally in meteorites, though very rarely and in very small amounts. Being the discoverer of SiC, Acheson was the first to synthesize SiC by passing an electric current through a mixture of clay and carbon. Today, SiC is still produced via a solid state reaction between sand (silicon dioxide) and petroleum coke (carbon) at very high temperatures in electric arc furnaces.
Several processes exist for making the SiC material. The most basic and simple process is combining sand and carbon in a temperature of about 1600 to 2500° C. Other processes for the synthesis of SiC include chemical vapor deposition and pyrolysis of organic SiC precursors such as polycarbosilane polymers. Conventional processes for the fabrication of near-net shape dense SiC components such as pressureless sintering and hot pressing, and the like are difficult to operate and control and are cost-prohibitive.
Conventional ceramic processes, such as pressureless sintering, for the manufacturing of near-net shape SiC articles are becoming costly due to the increased complexity of some of these SiC components. To produce a SiC article, the starting SiC powder is processed in the form of dry powder or a slurry followed by shaping, firing and final machining. The shaping or molding process requires the mold or die to be designed to accommodate for the shrinkage that takes place during the high temperature firing. The shrinkage of the SiC article is crucial to achieve the required densification or sintering. Accordingly, for every SiC article to be designed and manufactured, a special mold needs to be designed and made. Mold designs require the knowledge of shrinkage behavior of the material, which is a function of the SiC particle size distribution, chemistry and rheology of the slurry, firing temperature, heating and cooling rates, etc. The effects of these parameters need to be determined experimentally until enough information is available for a suitable mold to produce a specific near-net shape SiC article. Furthermore, the mold has a limited lifetime due to erosion resulting from each use that would cause dimensional changes of the mold, which in turn result in unpredictable dimensions of the sintered SiC product. In addition, producing a slurry with high solids loading and controlled chemistry and rheology requires the use of a bimodal SiC powder, where coarse particulates are mixed with fine powder. The use of coarse SiC particulates presents many problems such as poor thermal shock resistance of the material and poor isotropy. Recent technical advances indicate that monolithic and composite materials with fine microstructure possess superior and more uniform properties.
Prior art processes for manufacturing dense SiC articles are difficult to operate and are expensive. A need exists for a process that is capable of producing SiC articles that have reduced porosity to improve the chemical, mechanical, and thermal properties associated with SiC articles. It would be advantageous for the process to be simple to operate and be cost effective.
Recent studies have led to the development of a new SiC manufacturing process for the fabrication of near net shape SiC articles starting with near net shape graphite articles. The new process is known as the chemical vapor reaction (CVR) process. The CVR process has the capability of producing very complex shapes of SiC articles at a much lower cost than the conventional processes described above. However, SiC articles produced by the CVR process contain some porosity that could be detrimental to the chemical, mechanical and thermal properties of the SiC product. It would be advantageous to overcome this residual porosity. It would be advantageous to convert such porosity into a second phase of SiC.